Blacks and dingys: confusing cutworms

Each spring, seedling corn is attacked by a variety of caterpillars. Insect identification is the first step in determining whether there is a potential problem. Two of the most commonly confused insects are black cutworms and dingy cutworms.

Black cutworms are less than 0.5 inch in length and feed on leaves, whereas larger larvae can cut or drill plants. Blacks cause almost all cutworm damage to corn. Cutting can occur below the surface when soil is dry, or above ground, when soil is wet and tight around the plant. Cutting rarely occurs after the fifth true-leaf stage, but cutworms may drill into the side of larger stalks. The black cutworm does not overwinter in Iowa. Eggs are laid in the spring, when adults fly into Iowa from southern states.

Dingy cutworms eat leaves on young corn plants and injury is similar to that caused by black cutworms. Dingys rarely cut corn; therefore, it is important to determine if leaf feeding is from black or dingy cutworms. Dingy cutworms hatch in the fall, and overwinter in Iowa as partially grown larvae. Larvae found in corn during late April and early May are usually dingys and not black cutworms.

Black and dingy cutworms are separated by two distinctive features: skin texture and relative tubercle (wart) diameter. Blacks have grainy skin like sandpaper, whereas dingys have smooth skin. You will need a good hand lens to see this feature. Identification also is based on the size of the four tubercles along the top center of each body segment. On the dingy cutworm, these tubercles are about the same diameter. On the black cutworm, the inside pair of tubercles (or the pair closest to the head) is about half the diameter of the outside pair. Look at the circles in the photograph to find the location of these tubercles.